Preparation and Characterization of Crystalline Thin Films Belonging in the Cu-In-S System for High Conversion Efficiency Solar Cells

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The preparation and characterization of crystalline thin films belonging in the Cu-In-S system were performed as the basic study for high conversion efficiency solar cells. The films were grown on Si (001) wafers using a multisource evaporation method.Thin films of CuInS_2, CuIn_5S_8 and mixture of these were epitaxially grown when the In source temperature was varied. XRD and RHEED observation showed that the CuInS_2 film did not crystallize in the chalcopyrite structure, but in the Cu-Au structure with the c-axis normal to the substrate coexisting with the sphalerite structure. The CuIn_5S_8 film crystallized in the relaxed spinel structure with fairly good orientation. No evidence of CuIn_3S_5 was found from the XRD patterns.The Cu rich CuInS_2 films deposited at 500℃ crystallized in the chalcopyrite structure coexisting with the Cu-Au structure and/or sphalerite structure. In films with the stoichiometric composition, however, the chalcopyrite phase was not found.The photoluminescence (PL) spectra of films grown at 500℃ including chalcopyrite phase were measured at near 20K for the first time. In addition to the donor-acceptor pair emission, the weak and broad exitonic emissions were also observed. The photon energy and broadening of the emission are explained as the superposition of different emissions caused by the different crystal structure.The results obtained in this study will be useful for the growth of CuInS_2 films without extra phases except for the chalcopyrite structure which have never been grown.